Method of centrifugal casting



Patented May 13, 1947 METHOD OF CENTRIFUGAL CASTING Durward E. Breakeiield, Watertown Arsenal, and James L. Martin, Newton, Mass.

No Drawing. Application December 1, 1942, Serial No. 467,536

2 Claims. (01. 22-2005) (Granted under the act of March 3, 1883, as

The invention described herein may be manufactured and used by or for the Government fo governmental purposes without the payment to us of any royalty thereon.

The present invention relates to a process of centrifugal casting of objects having a length substantially greater than the transverse dimension, or more specifically, thick walled cylinders or conical shapes of a substantial length and having a wall thickness equal to or greater than the inside diameter of the article as cast."

In the centrifugal casting of many articles, such as pipes, the thickness of the layer of metal to be formed is relatively thin compared to the other dimensions of the article, and the solidification of the metal may proceed along several directions due to the heat extraction through the mold walls and radiation of heat from the exposed surface of the cast metal. The section being thin, the heat extraction in the diiIerent directions does not allow for substantially complete solidification of the two surfaces materially ahead of the center portion of the casting. If the casting has a substantial thickness and the two surface portions are allowed to solidify first, then the last metal to solidify will be in the midwall which will produce piping and cracks.

In the casting of an object that has a substantial thickness, the 'solidflcation must take place unidirectionally and under control in order to avoid segregation and cold shuts. If the solidification is to take place unidirectionally, the heat must be abstracted in one direction and'therefore, the use of a refractory lined mold is eliminated from consideration. It is obvious that if the solidificationis not restricted to a single direction that freezing will take place at various points and a dense structure is not produced.

The following of the practice of centrifugal casting art as applied to pipes and similar objects is not applicable to the casting of objects that have a transverse dimension equal to or greater than the inside of the cast article, as for example ferrous gun barrels. The greater mass of metal which is to be formed into the cast articles presents radically different considerations than that of a. thin article as above outlined. The solidification of the metal must be made to proceed from the mold to the center of the cast article. In order to accomplish this solidification the mold must be of such type that heat can be abstracted through the walls of the same and the use of a refractory lined mold, as is common in centrifugal casting, is eliminated from consideration as a, substantial thickness of insulation would so amended April 30, 1928; 370 O. G. 757) retard the rapid transference of heat that a unidirectional solidification would not take place. The metal to be solidified must be so introduced into the mold that the temperature of the same is substantially constant, and solidified as introduced so that segregation is held to a minimum. If segregation is allowed to take place by slow cooling, the alloys and non-metallics collect on the surface or in higher concentration in the surface metal and form an insulating layer which retards the extraction of heat from subsequently applied layers of metal. In the casting of objects having a thin wall, as for example pipes,

'the cooling which takes place on the exposed surface of the cast metal represents a substantial portionof the cooling taking place, and the total extraction of heat from the metal may not be substantially varied by segregation retarding the extlraction of heat through the insulated mold wa ls.

While solidification of a casting must proceed as rapidly as possible to prevent segregation, it is clear that each layer applied must not solidify at such a rate as to prevent the succeeding layers' from fusing together. If the succeeding layers fail to fuse together cold shuts are formed. The main factor which controls the fusing together of the succeeding layers of the cast metal is the rate of heat extraction. The rate of heat extraction is influenced by the temperature of the metal introduced, the chill ratio, the speed of rotation of the mold and the rate of introduction of metal into the mold. The chill ratio and the speed of rotation vary the rate of heat extraction. 7

The speed of rotation of the mold, the temperature of the molten metal and the rate of introduction of metal into the mold are factors which vary the amount of heat added. The speed of rotation of the mold acts in two ways, namely, the greater the speed of rotation the greater the cooling due to air cooling of the mold and the speed varies the rate distribution of heated (molten) metal through the mold. By holding the temperature of the molten metal at a substantially constant temperature in the usual range for casting and controlling the other factors above named in a critical range, the centrifugal casting of an article of the type described can be formed with a minimum of segregation and with the successive layers of metal deposited in the mold properly and completely fused together regardless of the thickness of the casting to be formed.

In order to secure the controlled unidirectional solidification, a mold must be so selected thatthe volumetric relation between the chill mold and the casting alone falls within a specified range. For convenience of consideration the volumetric relation of the mold is expressed as a ratio between cross-sectional area of the mold and casting and such ratio is referred to as the chill ratio. In the case of a long mold. it is sometimes necessary to .use a reinforcing rib on the mold exterior and the volumetric content of metal in said extension may be considered for purposes of computation as being distributed over the whole surface. The chill ratio shall be so controlled or regulated that the chill ratio is at least 2.5/1. The upper limit of the chill ratio is controlled by practical considerations and the resultant prohibitive mold sizes necessary to produce a high chill ratio. For practical purposes the chill ratios of 2.5/1 to 6/1 have been found satisfactory. The preferred mold is constructed with a chill ratio of 5/1 to 6/1 at the pouring end of the mold where the hot metal first strikes the mold and a chill ratio 2.5/1 to 3.5/1 at the non-pouring end of the mold. The chill ratio may be varied in said range depending upon the length of the casting and the chill ratio of the portions of the mold between the pouring end and non-pouring end may be varied directly as the length of the mold between the said limits.

In chill molds of any type the heat from the casting is of course absorbed by the mold through conduction and then transferred to the surrounding atmosphere by radiation. The heat absorbed by the mold will be transferred more rapidhr through conduction from the inner surface of the mold to the outer surface than it will be by radiation from the mold to the surrounding atmosphere; therefore, the larger the volume of the mold, or the thicker the wall of the mold, the greater will be the chilling action on the casting. Thus it follows that if the volumetric relation or chill ratio of the mold to a given casting be changed, them the chilling action willlikewise be so changed. By increasing the chill ratio for a given size casting the metal will solidify more quickly, and therefore, less segregation of the alloying elements will take place than would be the case under conditions of slower solidification.

The rate of introduction of metal into the mold, as noted before, must be correlated with the other factors previously mentioned, or the rate of metal introduction will in effect change the other factors in such a way as to negative the control so exercised. The rate of introduction of molten metal into the mold regulates the heat introduced into the mold, and also, the depth of the layer of metal distributed by centrifugal force over the casting surface of the casting being formed. With the introduction of metal at a, substantial rate, the metal already cast, if any, and the metal of the mold is heated at a rate proportional to the rate of introduction of molten metal, with the result that the mold wall is heated to such a temperature that a proper temperature differential between the metal being introduced and the temperature of the mold or solidified metal previously deposited, is not established. With a lowered temperature differential, the effective chill ratio is also lowered and the metal is not solidified in the required time interval to prevent segregation as desired. Conversely, if the metal is introduced too slowly, then the cast metal is solidified so rapidly that subsequently applied layers are not fused together properly and cold shifts are formed. The pouring rate must be so controlled that the metal is introduced at a rate 4 of 5 to 15 pounds per second and the variation in the range should vary directly with the chill ratio.

The rotation of the mold must be at such a speed that the metal is distributed over the surface which is to receive the molten metal in a minimum of time. In other words the cast metal must be carried to its solidifying point as quickly as possible in order that the heat added by the molten metal will be equally distributed over the vary the rotation speed in this range in order to accomplish the proper distribution are the length of the mold used, and the diameter of desired size of opening in the finished casting. The longer the length of the mold, and the smaller the opening in the finished casting will demand a greater speed to distribute the metal in the mold.

A further influence of a high rotation speed on the casting is the control of the shape of the opening in the finished casting. In many cases it is desirable to have the casting formed with an opening of the same size throughout the length of the casting. If the casting is to be used as a fluid conduit for example, it is imperative that the opening in the casting have an inner surface that is smooth and without constrictions throughout the length. A further advantage of a controlled size of the inner surface of the casting is the production of a casting that has a substantially constant wall thickness. If the casting is of a conical shape on the exterior the inner surface will be constant and the wall thickness will vary in a straight line relation as the exterior surface varies. The use of a high rotation speed distributes the metal in the molten state in waves or sheets throughout the casting cavity and prevents the formation of uneven surfaces on the interior of the casting.

Examples of the effect of the rotative speed of v the mold on the casting dimensions is given in the following table:

Outside Internal Diameter Diameter Le m1 n er- Inches Pour- Pcurence Pour- Pouring mg mg in Inches 1? End, Eng, Inc es Inches Inches Inches 130 12 6 /1; 3% 2% 900 if: 3% 2% 900 130 12 6 1% 316 1% 1, 1% 3% 1% 1,100 1% 3% 1% 1,100 12 1% l M 200 1V. fie 9ie 1,200 2% 2% V4 1,200 1% life m 1,200

By using a rotation speed of 1200 R. P. M. or above the unevenness of the internal surface can be minimized.

In the casting of a cylindrical object the metal may be introduced from either end, while with a casting which is of conical shape, it is preferable to introduce the metal at the large end. On pouring metal into the small end of a conical mold, the metal does not proceed to the nonpouring end in waves or sheets as is required, but is thrown violently to the large end and the proper balance of heat control is prevented. If it is desired to introduce the metal into the small end of a conical mold, the mold may be tilted so 5 that the axis of the mold is inclined at an angle to the horizontal of about to 22, and the angle used depends upon the departure of the mold from a cylindrical shape.

While a refractory lined mold is not used, it is within the purview of this invention to use a mold wash which is well known by those skilled in the art to prevent sticking of the casting to the mold.

We claim:

1. The method of centrifugally casting ferrous articles having a radial thickness at least equal to the internal diameter of the casting which comprises introducing molten metal at a substantially constant temperature at a rate of to 15 pounds per second into a, mold rotating at a speed of 900 to 1500 R. P. M. and whose chill ratio is 2.5/1 to 6/1, the pouring rate varying directly with the chill ratio.

2. The method of centrifugally casting a blank for a ferrous gun barrel, said blank having a radial thickness at least equal to the internal diameter of the casting, which comprises pouring molten metal at a rate of 5 to 15 pounds per second into a chill mold whose chill ratio at the pouring end is 5/1 to 6/1 and the chill ratio at the nonpourlng end is 2.5/1 to 3.5/1, the said pouring rate varying directly with said chill ratio, the

said mold rotating about a substantially horizontal axis at a speed of 900 to 1500 R. P. M.

DURWARD E. BREAKEFIELD JAMES L. MARTIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Foundry Trade Journal, Sept. 3, 1931, pages 145, 145, 147.

Transactions of American Society for Steel Treating, vol. 18, pages 212 to 240, inclusive. 

